The present invention relates generally to gas turbine engines, and, more specifically, to turbine blade cooling.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel and ignited in a combustor for generating hot combustion gases. The gases flow through turbine stages which extract energy therefrom for powering the compressor and producing useful work, such as powering a fan for propelling an aircraft in flight.
A turbine stage includes a row of turbine blades extending radially outwardly from a supporting rotor disk. Each blade includes an airfoil over which the combustion gases flow for extracting energy therefrom. The airfoil is hollow and is provided with air bled from the compressor for use as a coolant in cooling the blade during operation.
Maximum efficiency of the turbine is obtained by closely positioning the radially outer tip of the airfoil adjacent a surrounding stationary turbine shroud for minimizing combustion gas leakage therebetween. However, differential thermal expansion and contraction between the blade tips and turbine shroud can cause rubbing therebetween. To accommodate this rubbing, the tip of the airfoil includes a squealer tip rib around its perimeter extending outwardly from a tip cap enclosing a coolant flow channel inside the airfoil. The squealer rib may be closely positioned adjacent the shroud and limits rubbing therebetween to the surface area of the rib itself.
The squealer rib is thusly exposed on three sides to the hot combustion gases and is difficult to cool, and correspondingly affects useful blade life. The airfoil tip is typically cooled by providing tip holes through the tip cap which discharge a portion of the coolant into the tip cavity defined above the tip cap. And, inclined film cooling holes may extend through the concave or pressure sidewall of the airfoil just below the tip cap to provide film cooling air which bathes the pressure side portion of the tip with film cooling air.
However, cooling of the squealer rib is limited in effectiveness, and thermal gradients and s tress therefrom are created which also affect blade life. The exposed squealer rib runs hotter than the airfoil sidewalls, with the tip cap therebetween running cooler. Tip cooling must therefore be balanced against undesirable thermal gradients.
Accordingly, it is desired to provide a turbine blade having improved airfoil tip cooling.